
<!DOCTYPE html>

<html lang="en">
  <head>
    <meta charset="utf-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" /><meta name="generator" content="Docutils 0.19: https://docutils.sourceforge.io/" />

  <title>variable_watch &mdash; CMake 3.27.4 Documentation</title>

    <link rel="stylesheet" type="text/css" href="../_static/pygments.css" />
    <link rel="stylesheet" type="text/css" href="../_static/cmake.css" />
    
    <script data-url_root="../" id="documentation_options" src="../_static/documentation_options.js"></script>
    <script src="../_static/jquery.js"></script>
    <script src="../_static/underscore.js"></script>
    <script src="../_static/_sphinx_javascript_frameworks_compat.js"></script>
    <script src="../_static/doctools.js"></script>
    <script src="../_static/sphinx_highlight.js"></script>
    
    <link rel="shortcut icon" href="../_static/cmake-favicon.ico"/>
    <link rel="index" title="Index" href="../genindex.html" />
    <link rel="search" title="Search" href="../search.html" />
    <link rel="next" title="while" href="while.html" />
    <link rel="prev" title="unset" href="unset.html" />
 

  </head><body>

    <div class="related" role="navigation" aria-label="related navigation">
      <h3>Navigation</h3>
      <ul>
        <li class="right" style="margin-right: 10px">
          <a href="../genindex.html" title="General Index"
             accesskey="I">index</a></li>
        <li class="right" >
          <a href="while.html" title="while"
             accesskey="N">next</a> |</li>
        <li class="right" >
          <a href="unset.html" title="unset"
             accesskey="P">previous</a> |</li>
  <li>
    <img src="../_static/cmake-logo-16.png" alt=""
         style="vertical-align: middle; margin-top: -2px" />
  </li>
  <li>
    <a href="https://cmake.org/">CMake</a> &#187;
  </li>
  <li>
    <a href="../index.html">3.27.4 Documentation</a> &#187;
  </li>

          <li class="nav-item nav-item-1"><a href="../manual/cmake-commands.7.html" accesskey="U">cmake-commands(7)</a> &#187;</li>
        <li class="nav-item nav-item-this"><a href="">variable_watch</a></li> 
      </ul>
    </div>  

    <div class="document">
      <div class="documentwrapper">
        <div class="bodywrapper">
          <div class="body" role="main">
            
  <section id="variable-watch">
<span id="command:variable_watch"></span><h1>variable_watch<a class="headerlink" href="#variable-watch" title="Permalink to this heading">¶</a></h1>
<p>Watch the CMake variable for change.</p>
<div class="highlight-cmake notranslate"><div class="highlight"><pre><span></span><span class="nf">variable_watch(</span><span class="nv">&lt;variable&gt;</span><span class="w"> </span><span class="p">[</span><span class="nv">&lt;command&gt;</span><span class="p">]</span><span class="nf">)</span>
</pre></div>
</div>
<p>If the specified <code class="docutils literal notranslate"><span class="pre">&lt;variable&gt;</span></code> changes and no <code class="docutils literal notranslate"><span class="pre">&lt;command&gt;</span></code> is given,
a message will be printed to inform about the change.</p>
<p>If <code class="docutils literal notranslate"><span class="pre">&lt;command&gt;</span></code> is given, this command will be executed instead.
The command will receive the following arguments:
<code class="docutils literal notranslate"><span class="pre">COMMAND(&lt;variable&gt;</span> <span class="pre">&lt;access&gt;</span> <span class="pre">&lt;value&gt;</span> <span class="pre">&lt;current_list_file&gt;</span> <span class="pre">&lt;stack&gt;)</span></code></p>
<dl class="simple">
<dt><code class="docutils literal notranslate"><span class="pre">&lt;variable&gt;</span></code></dt><dd><p>Name of the variable being accessed.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre">&lt;access&gt;</span></code></dt><dd><p>One of <code class="docutils literal notranslate"><span class="pre">READ_ACCESS</span></code>, <code class="docutils literal notranslate"><span class="pre">UNKNOWN_READ_ACCESS</span></code>, <code class="docutils literal notranslate"><span class="pre">MODIFIED_ACCESS</span></code>,
<code class="docutils literal notranslate"><span class="pre">UNKNOWN_MODIFIED_ACCESS</span></code>, or <code class="docutils literal notranslate"><span class="pre">REMOVED_ACCESS</span></code>.  The <code class="docutils literal notranslate"><span class="pre">UNKNOWN_</span></code>
values are only used when the variable has never been set.  Once set,
they are never used again during the same CMake run, even if the
variable is later unset.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre">&lt;value&gt;</span></code></dt><dd><p>The value of the variable.  On a modification, this is the new
(modified) value of the variable.  On removal, the value is empty.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre">&lt;current_list_file&gt;</span></code></dt><dd><p>Full path to the file doing the access.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre">&lt;stack&gt;</span></code></dt><dd><p>List of absolute paths of all files currently on the stack of file
inclusion, with the bottom-most file first and the currently
processed file (that is, <code class="docutils literal notranslate"><span class="pre">current_list_file</span></code>) last.</p>
</dd>
</dl>
<p>Note that for some accesses such as <span class="target" id="index-0-command:list"></span><a class="reference internal" href="list.html#append" title="list(append)"><code class="xref cmake cmake-command docutils literal notranslate"><span class="pre">list(APPEND)</span></code></a>, the watcher
is executed twice, first with a read access and then with a write one.
Also note that an <span class="target" id="index-0-command:if"></span><a class="reference internal" href="if.html#defined" title="if(defined)"><code class="xref cmake cmake-command docutils literal notranslate"><span class="pre">if(DEFINED)</span></code></a> query on the variable does not
register as an access and the watcher is not executed.</p>
<p>Only non-cache variables can be watched using this command.  Access to
cache variables is never watched.  However, the existence of a cache
variable <code class="docutils literal notranslate"><span class="pre">var</span></code> causes accesses to the non-cache variable <code class="docutils literal notranslate"><span class="pre">var</span></code> to
not use the <code class="docutils literal notranslate"><span class="pre">UNKNOWN_</span></code> prefix, even if a non-cache variable <code class="docutils literal notranslate"><span class="pre">var</span></code>
has never existed.</p>
</section>


            <div class="clearer"></div>
          </div>
        </div>
      </div>
      <div class="sphinxsidebar" role="navigation" aria-label="main navigation">
        <div class="sphinxsidebarwrapper">
  <div>
    <h4>Previous topic</h4>
    <p class="topless"><a href="unset.html"
                          title="previous chapter">unset</a></p>
  </div>
  <div>
    <h4>Next topic</h4>
    <p class="topless"><a href="while.html"
                          title="next chapter">while</a></p>
  </div>
  <div role="note" aria-label="source link">
    <h3>This Page</h3>
    <ul class="this-page-menu">
      <li><a href="../_sources/command/variable_watch.rst.txt"
            rel="nofollow">Show Source</a></li>
    </ul>
   </div>
<div id="searchbox" style="display: none" role="search">
  <h3 id="searchlabel">Quick search</h3>
    <div class="searchformwrapper">
    <form class="search" action="../search.html" method="get">
      <input type="text" name="q" aria-labelledby="searchlabel" autocomplete="off" autocorrect="off" autocapitalize="off" spellcheck="false"/>
      <input type="submit" value="Go" />
    </form>
    </div>
</div>
<script>document.getElementById('searchbox').style.display = "block"</script>
        </div>
      </div>
      <div class="clearer"></div>
    </div>
    <div class="related" role="navigation" aria-label="related navigation">
      <h3>Navigation</h3>
      <ul>
        <li class="right" style="margin-right: 10px">
          <a href="../genindex.html" title="General Index"
             >index</a></li>
        <li class="right" >
          <a href="while.html" title="while"
             >next</a> |</li>
        <li class="right" >
          <a href="unset.html" title="unset"
             >previous</a> |</li>
  <li>
    <img src="../_static/cmake-logo-16.png" alt=""
         style="vertical-align: middle; margin-top: -2px" />
  </li>
  <li>
    <a href="https://cmake.org/">CMake</a> &#187;
  </li>
  <li>
    <a href="../index.html">3.27.4 Documentation</a> &#187;
  </li>

          <li class="nav-item nav-item-1"><a href="../manual/cmake-commands.7.html" >cmake-commands(7)</a> &#187;</li>
        <li class="nav-item nav-item-this"><a href="">variable_watch</a></li> 
      </ul>
    </div>

    <div class="footer" role="contentinfo">
        &#169; Copyright 2000-2023 Kitware, Inc. and Contributors.
      Created using <a href="https://www.sphinx-doc.org/">Sphinx</a> 5.3.0.
    </div>
  </body>
</html>